Field of the Invention
The present invention relates to a point light source, a planar light source device, and a display device.
Description of the Background Art
Japanese Patent No. 4956977 and Japanese Patent Application Laid-Open No. 2011-113648 each disclose a point light source using a light-emitting diode (LED) (hereinafter may be referred to as a LED point light source) as a light-emitting element.
In the LED point light source of Japanese Patent No. 4956977, the LED on a substrate is covered by resin. The resin has a first plane including one long side of the substrate, and a second plane that faces the first plane and includes the other long side of the substrate. The resin is semicircular when viewed from the first plane side and the second plane side. This semicircular curved surface forms a light-emitting surface (lens forming surface) of the LED point light source. The first and the second planes are parallel to a main surface of a light guide plate. The light-emitting surface is orthogonal to the first and the second planes, and the main surface of the light guide plate.
In the LED point light source of Japanese Patent Application Laid-Open No. 2011-113648, resin that covers the LED includes a rectangular cuboid base and a protruding portion of an elliptical hemisphere protruding forward from the front surface of the base.
In the LED point light source of Japanese Patent No. 4956977, a portion of light generated in the LED travels straight toward a light-emitting surface of resin that exists ahead. Meanwhile, most light travels to the first and the second resin planes, and arrives at the light-emitting surface while being reflected between these two planes. The first plane is mounted on a substrate, which is adapted to facilitate reflection on the first plane. The second plane is provided with a reflective member, which is adapted to facilitate reflection on the second plane.
However, for example, from among light reflected by the first and the second planes and arrives at the light-emitting surface at a large incidence angle, some light may be totally reflected by the light-emitting surface and may not be emitted from the light-emitting surface. This will lower light extraction efficiency (in other words, LED light utilization efficiency).
In addition, when reflection is repeated by the first and the second planes, an optical path in the resin will become longer. Accordingly, even if the light reflected by the first and the second planes is finally emitted from the light-emitting surface, only light intensity attenuated in proportion to the number of reflection and the optical path length is obtained. Also from this viewpoint, the light extraction efficiency will become lower.
Referring to a longitudinal section (cross-section which passes through the LED and is orthogonal to the first and second planes of resin, in other words, cross-section which passes through the LED and is orthogonal to the upper and lower main surfaces of the light guide plate), light is emitted at various angles from the light-emitting surface. For example, when light reflected by the first and the second planes is emitted from the light-emitting surface at a large emitting angle, the light will have a small incidence angle with respect to the main surface of the light guide plate after entering the light guide plate. Then, the light will travel within the light guide plate while repeating reflection between the upper and lower main surfaces of the light guide plate. Since repeated reflection and resulting increase in the optical path length cause attenuation of light intensity, however, the light cannot travel far in the light guide plate.
Accordingly, the light utilization efficiency becomes lower in the light guide plate, in other words, in a planar light source device. As a result, the entire brightness becomes lower. Moreover, places far from the LED point light source become darker, generating brightness unevenness.
In the LED point light source of Japanese Patent Application Laid-Open No. 2011-113648, as described above, the resin that covers the LED includes the rectangular cuboid base and the protruding portion provided in front of the base. Since the protruding portion from which light is emitted has a shape of an elliptical hemisphere, light that is totally reflected by the protruding portion and is not emitted may be reduced compared with the LED point light source of Japanese Patent No. 4956977.
However, other problems that describe the LED point light source of the Japanese Patent No. 4956977 are considered to be applicable to the LED point light source of Japanese Patent Application Laid-Open No. 2011-113648.